One probably has little to do with the other. People live their lives and define their sexuality as they do based on a lot of factors. Mental illness and personality disorders most likely play a part, but are far from the whole story. Just speaking for myself, there are dozens, if not hundreds, of factors that played a part in me choosing polyamory. I'd imagine for most people, there are at least a few, and no one thing was the deciding factor.

If there is any correlation here at all, it's most likely to do with openness. Polyamory is seen as negative by society in general, as are mental disorders. If one is polyamorous and open about it, they are probably more likely to be open about other things society sees as negative as well. Pretty much any group of people that is considered non-mainstream seems to show a higher percentage of mental disorders than do mainstream groups.